This invention relates generally to the control of pollutants during a combustion process. More particularly, this invention relates to a technique of using high permeability filter bags in a barrier filter, such as a bag house.
FIG. 1 illustrates a flue gas treatment system 10 constructed in accordance with the prior art. The system 10 is described in U.S. Pat. No. 5,158,580, which is assigned to the assignee of the present invention, and which is incorporated by reference herein. The flue gas treatment system 10 treats a flue gas exiting a boiler 12. The boiler 12 is of the type used in a utility fossil-fuel-fired power plant. Fuel supply 18 may be, for example, coal, oil, refuse derived fuel (RDF) or municipal solid waster (MSW). Boiler 12 also receives air 20 through inlet duct 22. Boiler 12 functions to combust the fuel 14 with air 20 to form flue gas 24, which exits boiler 12 by means of outlet duct 26. Boiler 12 also has a water inlet pipe 28 and a steam outlet pipe 30 for removing heat in the form of steam from boiler 12 generated by the combustion of fuel 14 with air 20.
Flue gas 24 is comprised of components of air and the products of combustion in gaseous form which include: water vapor, carbon dioxide, halides, volatile organic compounds, trace metals vapors, and sulfur and nitrogen oxides and the components of air such as oxygen and nitrogen. Flue gas 24 also contains particulate comprising unburned and partially combusted fuel which includes inorganic oxides of the fuel (known as flyash), carbon particles, trace metals, and agglomerates. Flue gas 24 may also contain particulate generated by the addition of removal agents 19 for sulfur oxide and other gas phase contaminates, such as halides and trace metal vapors. The removal agents 19 may be added into duct 21, duct 26, or into reactor vessel 17 by way of duct 23. Ducts 21, 23 and 26 may also convey solid materials if required for the selected removal agents 19 for the respective duct. Examples of sulfur oxide and other gas phase contaminate removal agents 19 include calcium carbonates, oxides and hydroxides, and sodium carbonates and bicarbonates. The particles or particulate in flue gas 24 can vary considerably in size, shape, concentration and chemical composition.
Flue gas 24 passes through duct 26 through reactor vessel 17 and through duct 27 as flue gas 25 to an inlet of electrostatic precipitator 34, which functions to charge and collect particles on electrodes within the electrostatic precipitator 34. Reactor vessel 17 may facilitate the chemical reaction of removal agents 19 with flue gas 24 to provide treated flue gas 25. Electrostatic precipitator 34 may remove, for example, from 90-99.9% of the particles and/or particulate. Therefore, flue gas 24 exits electrostatic precipitator 34 as treated flue gas 36 entering outlet duct 38. Treated flue gas 36 has roughly from 0.1-10% of the particulate or particles contained in the original flue gas 24 and also contains a certain amount of electrostatic charge which was transferred to it from the electrostatic precipitator 34. These particles were not collected within the electrostatic precipitator, but exited at outlet duct 38.
The particle concentration in the flue gas 36 exiting the electrostatic precipitator 34 is reduced significantly by the precipitator and contains a residual charge imparted by the precipitator. Particulate 36 leaving the electrostatic precipitator may lose charge if there is a long path between the electrostatic precipitator 34 and the barrier filter 44. To prevent this problem, a pre-charging unit 40 may be used. Gas 36 enters pre-charging unit 40 through inlet duct 38. The pre-charging unit 40 operates to charge particulate and then deliver it to a barrier filter 44.
Examples of acceptable barrier filters 44 include baghouses of the pulse-jet type, reverse flow, or shake-deflate type for periodically removing the dust cake accumulated on the surface of the bag filter. Prior art barrier filters are formed with bags having a nominal air permeability of 25 to 50 acfm/sq ft (actual cubic feet per minute of air flow per square foot of filter surface area at a pressure drop of one half inch water).
The flue gas 48 exiting barrier filter 44 passes through outlet duct 50, through fan 52 and through duct 54 to the inlet of smoke stack 46. Flue gas 48 exits smoke stack 46 as gas 58, which in turn mixes with the ambient air or atmosphere.
Fan 52 functions to overcome the additional pressure drop required to draw flue gas 48 across the barrier filter 44. Fan 52 also functions to draw flue gases 36 and 24 from electrostatic precipitator 34 and boiler 12 respectively. Fan 52 further functions to move flue gas 48 through duct 54 and out of smoke stack 46 as flue gas 58.
One problem associated with the system 10 is that the bags used in the barrier filter 44 have a relatively short life. Chemical degradation of the bag material and high pressure drop problems shorten bag life significantly. The causes for the degradation of the bag material are frequently unknown. For example, the causes for the degradation of bag material formed of RYTON (sold by American Fiber and Yarn) in the flue gas are not fully understood. New bag materials, such as TEFLON and glass, appear to provide improved resistance to chemical degradation. However, felted fabrics made of TEFLON are very expensive and both high efficiency TEFLON and glass filter felts still have high pressure drop problems.
The high pressure drop problem can be traced to the relatively poor efficiency of the existing ESP 34, which leads to high dust loading in the baghouse (i.e., barrier filter 44) and to the high air-to-cloth ratio associated with the baghouse. Thus, the high efficiency conventional felted RYTON filter needs to be cleaned frequently, leading to eventual bag blinding (i.e., penetration of dust into the felted fabric).
In view of the foregoing, it would be highly desirable to provide an improved barrier filter for use in a flue gas treatment system.
The invention includes a method of treating flue gas. Particulate is removed from the flue gas with a first particulate removal technique to produce treated flue gas. The treated flue gas is then applied to a high permeability barrier filter to remove additional particulate from the treated flue gas. The high permeability filter includes high permeability filter bags with air permeability greater than 75 actual cubic feet per minute of air flow per square foot of filter surface area at a pressure drop of one half inch water.
The invention also includes a flue gas treatment apparatus. The flue gas treatment apparatus includes a combustor to generate a flue gas. An electrostatic precipitator is connected to the combustor to remove particulate from the flue gas. A barrier filter is connected to the electrostatic precipitator to remove particulate from the flue gas. The barrier filter includes high permeability filter bags with air permeability greater than 75 actual cubic feet per minute of air flow per square foot of filter surface area at a pressure drop of one half inch water.
The fabric filter of the invention is easier to clean, provides reduced pressure drop, and is not subject to blinding. The filter fabric of the invention can also resist chemical degradation. The invention can be exploited in a variety of polishing baghouse operations where a baghouse is placed after a primary particulate collector to capture residual flyash.